Well logging, sometimes referred to as borehole logging, is the practice of making a detailed record (or a well log) of the geological formations penetrated by a borehole or a well. This log may be established either on a visual inspection of the samples brought to the surface (called geological logs) or on the physical measurements made by instruments lowered into the hole (called geophysical logs).

Geophysical well logs such as, drilling, completing, producing, or abandoning can be done during any phase of a well's history.